This invention relates to a collapsible support assembly, and more particularly, to improvements in a portable, collapsible derrick structure.
In various operations, such as building construction, boat launching and general warehouse moving, a need exists for a temporary hoist which can be transported to the work site, is quickly and easily erected from a collapsed, transporting position to an erected, operating position and is very stable during use. Several truck mounted arrangements are available which have an erectable derrick structure.
A portable assembly is shown in U.S. Pat. No. 4,068,762 entitled PICKUP TRUCK DERRICK, issued Jan. 17, 1978 to the present inventor. This patent discloses a collapsible boom and derrick hoist assembly having a base frame securable to the side rails of a pickup truck. A collapsible A-frame is pivotally mounted to the front end of the base frame. In the collapsed, folded position, the A-frame rests over the tailgate of the pickup truck. While in the erected position, the A-frame rises above the front end of the truck bed. The assembly also provides a brace for supporting the A-frame when the A-frame is in the erected position.
The hoist assembly includes an A-frame erection system for raising the A-frame to its operating position. In one embodiment, the erection system includes a winch and pulley arrangement located near the truck cab and a subassembly located near the tailgate for increasing the mechanical advantage of the winch. In another embodiment, the A-frame erection system has a pair of U-shaped rails attached to the side rails of the base frame. Each leg of the A-frame is pivotally mounted to a slider assembly which in turn is slidably mounted to the U-shaped rails. Pulling the sliders toward the front of the truck erects the A-frame to its vertical position near the front end of the truck bed.
The hoist assembly includes a foldable, two-piece boom which is supported by the A-frame and which is erected by means of a winch, cable and strut arrangement. The hoist assembly also includes a boom swinging mechanism for rotating the boom about its vertical axis, a boom elevating system for raising and lowering the boom, and a hoisting system for lifting a load.
U.S. Pat. No. 3,059,781 to Bender, entitled MATERIAL HANDLING DEVICE, issued Oct. 23, 1962 discloses a derrick elevating mechanism having a roller type structure. The elevating mechanism has a track spanned by a hydraulic powered roller assembly. A connecting arm extends between the roller assembly and a pivotal mast. Actuation of the hydraulic ram draws the roller up the track, causing the connecting arm to pivot the mast from a horizontal to a vertical position.
U.S. Pat. No. 3,797,672 to Vermette, entitled APPARATUS ATTACHABLE TO A TRUCK BODY OR THE LIKE FOR USE FOR HOISTING OR LIFTING, OR AS AN ELEVATED SUPPORT, issued on Mar. 19, 1974 discloses a latching mechanism used to lock a support brace to a derrick frame. The latching mechanism includes an upright auxiliary support brace having attached thereto an upper spring activated latch and a lower, gravity activated latch. As the derrick frame is raised to its elevated position, the upper latch engages the pin attached to the derrick to secure the derrick to the upright support. Simultaneously, the lower latch engages another pin attached to the derrick frame to lock the derrick in the upright position. In this arrangement, the operator must actually handle the latch mechanism to disengage it when it is in an upright position. Thus, the latch mechanism must be located near the base of the derrick frame so as to be within reach of the operator. Consequently, additional support braces and/or support wires must be used to support the upper portions of the derrick frame.
In the above derrick assemblies which include a hydraulic ram, the derrick erection system is connected to the derrick near the derrick pivot point. Consequently, considerable amounts of force must be developed by the erection assembly because the load on the ram is very high during the initial movement of the derrick due to the positioning of the erection system with respect to the derrick. In addition, in assemblies having a derrick fixed and pivoted to the base frame, the hoisting cable shortens due to the cable wrapping around its guide pulleys as the boom and A-frame are lowered. Cable shortening causes the cable to retract, and this retraction can result in the hooked end of the hoisting cable jamming into its guide pulley located at the end of the boom.
Therefore, a need exists for a cable system which will not retract as the derrick and boom are lowered. A need also exists for a bracke system which supports the upper portions of the derrick frame, which engages automatically as the derrick is raised, and which locks automatically once the derrick is erected. A need also exists for a derrick erection system which does not require auziliary erection systems or overhead cables and pulleys.